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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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154                                ALASKAN GLACIER STUDIES
As in 1899, 1905, and 1906 the surface of the Hidden Glacier was in 1909 remarkably free of debris so that in all the views one sees mainly clear ice. There was, in fact, even less d6bris than in previous years, for, with the advance of the glacier, there had been a lateral spreading and a rising on the valley side, as a result of which the lateral moraines had been destroyed. There was, therefore, an absence of the lateral moraines which formed a prominent feature of the glacier margin in previous years. The medial moraine was, however, still present and still over on the south side of the valley; but near the terminus it swung still farther south, and at the very end of the glacier became lateral. The fact that this medial moraine had retained its approximate position furnishes proof that the thrust by which the Hidden Glacier was broken and pushed forward did not come from the glacier tributary on the south by which this medial moraine is supplied. Had there been such a thrust from that glacier this medial moraine would of necessity have been pushed farther out into the glacier than it was in 1905. We assume, therefore, that the thrust came from either the east or the north.
The margins of the glacier were examined with some interest, because the ice had suddenly risen up over gravel and rock slopes, from which ice had for many years been absent. The two margins of the glacier were found to be quite different. Along the northern margin there was a band of barren moraine 200 to 300 feet in width, with no living vegetation but with many fragments of willow, alder and cottonwood. At its upper border, in a number of places, there was a well-defined ridge of bowlders, till, gravel, and plants shoved up by the advance of the glacier (PL LXXXIH). It is a perfect illustration of shoved, or push moraine, and in places was faulted and doubly ridged by the thrust. It wound up and down the hillside in irregular lobes, often rising highest on spurs rather than in adjacent gullies. The barren moraine (PI. LXXXVI, A) between this shoved moraine and the ice shows clearly how much recession there had been since the period of greatest lateral advance. In some places the marginal moraine was distinctly hummocky, especially in places where the hillside drainage, which in earlier years extended down to the outwash gravel plain, found itself interrupted in its progress by the newly-established base level of the advanced glacier. In these places the sediment which the hillside drainage was carrying had in part been deposited on the ice margin, and in some places there were patches of ice, still unmelted, in the barren zone from which elsewhere the glacier had completely receded. All along this margin there was evidence that the ice was rapidly withdrawing, with a thin wedgelike edge resting on the older till and gravels. Not all the barren zone above the present ice margin can be ascribed to recession of the glacier, for some of it doubtless represented the position of lingering snow banks, prevented from sliding down farther into the valley by the newly-imposed ice barrier. But this explanation can account for only a few areas of unusually broad barren zones, for in those areas which lie between the ice and in shoved moraine there can be no question but that ice extended up to the pushed area. Also in those portions of the barren zone where ice still remained beneath gravel deposited by marginal drainage we have positive proof that the barren zone was due to recent occupation by the glacier itself.
On the north side of the glacier the hillside drainage, now interrupted in its journey to the valley bottom by the presence of the glacier, united with the waters supplied by the melting of the glacier and flowed along a marginal course, in places flowing under the glacier for short distances, elsewhere flowing at or near the ice margin. In one place